liquid crystal elastomer (LCE) has become one of the hot research topics in the application fields of software robot , artificial muscles and flexible drivers due to its excellent programmability and fast reversible driving characteristics. These characteristics are derived from the coupling of its anisotropic liquid crystal molecules (LC) with the elastic crosslinking network. Clarifying the association between LCE mechanics and heat transfer anisotropy and microstructure is the key to achieving its controllable programming characteristics. Most existing research is committed to exploring the physical characteristics of LCE at low frequencies. However, some emerging high-frequency applications, such as 5G communication and RF system packaging, need to obtain the physical characteristics of LCE.
To address this problem, Tongji University Cang Yu Tokyo University of Technology Professor Morikawa
Among them, the anisotropy of elastic modulus is quite different at high and low frequencies. reflects the difference between local mesomorphic orientation and network chain orientation. By characterizing the strain dependence of elastic modulus, it was found that the mechanical Frédericksz transformation of LC is different from the soft elastic of most LCEs. Moreover, the thermal conductivity of is anisotropy caused by the combined effect of LC orientation and network structure. This work reveals thermal elastic anisotropy at different scales. The research results are titled "On the origin of elasticity and heat conduction anisotropy of liquid crystal elastics at gigahertz frequency", and are recently published in Nature Communications 2022, 13, 524.
Figure 1. Propagation of elastic waves in liquid crystal elastomer
LCE is a typical transverse isotropic material, and its independent elastic constant increases from 2 to 5 compared to isotropic materials. It is difficult to obtain a complete elastic constant through traditional destructive testing methods. The author used a Brillouin spectrometer to detect the propagation speed of giHz phonons in various directions inside the material, and used the Christoffel equation to analyze and obtain 5 elastic constants and the elastic modulus of anisotropy, Poisson's ratio , etc. Compared with the static Young's modulus anisotropy (E||/E⊥) obtained by tensile test, the values of high-frequency E||, BLS/E⊥, and BLS are lower and constant, and do not change with the crosslinking density and the degree of friction orientation during the preparation process. Since high-frequency anisotropy is related to mesomorphic molecular orientation, this result indicates a difference between local mesomorphic orientation and the larger-scale orientation of the network chain.
Figure 2 Changes in sound velocity with increasing strain
In addition, high-frequency sound velocity and its anisotropy are robust during the stretching process. When the stress stretches along the orientation direction of the liquid crystal elastomer, the orientation of its liquid crystal molecules does not change, so the speed of sound remains unchanged. However, when stretched along the vertical direction, the constant speed of sound is unexpected, because through the stress-strain diagram, the liquid crystal elastomer exhibits soft elasticity, in other words, the liquid crystal molecules will rotate continuously until they are oriented parallel to the stress direction. However, the surface of the robustness of the sound speed does not rotate continuously, but undergoes a mechanical Frédericksz transformation, that is, under a certain strain, the liquid crystal molecules rotate instantly, rather than a continuous process. The thermal conductivity of liquid crystal elastomers also exhibits anisotropy. However, compared with the thermal conductivity anisotropy of liquid crystal molecules, the thermal conductivity of liquid crystal elastomers and the anisotropy of phonon average free path are not only related to the structural anisotropy of liquid crystal molecules, but are also affected by the cross-linked network.
Summary: This work reveals the anisotropic characteristics of different length scales of liquid crystal elastomers, providing insights into the design of on-demand programming of liquid crystal elastomers and their high-frequency applications.
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Source: Frontiers of Polymer Science
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